SG-19789 licensing (#236)

Removed unused `xmlrpclib` 3rd party module.
Added software credits for 3rd party components.
Increased the thumbnail download retry attempts and interval.

Author: pscadding

.coveragerc

@@ -17,4 +17,3 @@ source=shotgun_api3
 omit=
     shotgun_api3/lib/httplib2/*
     shotgun_api3/lib/six.py
-    shotgun_api3/lib/xmlrpclib.py

.flake8

@@ -10,4 +10,4 @@
 
 [flake8]
 max-line-length = 120
-exclude = shotgun_api3/lib/httplib2/*,shotgun_api3/lib/six.py,shotgun_api3/lib/xmlrpclib.py,tests/httplib2test.py,tests/mock.py
+exclude = shotgun_api3/lib/httplib2/*,shotgun_api3/lib/six.py,tests/httplib2test.py,tests/mock.py